The present invention relates to an antenna device and, particularly, to a sleeve antenna.
As is commonly known, the sleeve antenna is a coaxial vertical-antenna, and a single, vertical half-wave radiator. The sleeve antenna does not have horizontal directivity. Such a sleeve antenna is used for eliminating coupling of a feeder with the antenna.
FIG. 1 shows an aspect of a sleeve antenna 1. The sleeve antenna 1 comprises a lower half 2 and an upper radiant-section 3. The lower half 2 is a metal sleeve through which a feeder 4 concentrically passes. As the feeder 4, a coaxial cable is used. The radiant section 3 has a length of quarter wavelength (λ/4), and is connected to a central conductor of the feeder 4.
When the coaxial cable is used as the feeder, a central conductor itself of the coaxial cable can be used as the radiant section 3. An outer conductor of the coaxial cable is electrically connected to the metal sleeve 2 by solder.
The following antenna device, while it is not the sleeve antenna, is known as a related art of the invention. Japanese Unexamined Patent Application Publication (JP-A) No. 2003-338708 discloses a small-size, lightweight, and inexpensive antenna device that can resonate with both of two types of wavelengths, λ1 and λ2.
As above, since the sleeve antenna does not have the horizontal directivity, it can exhibit certain horizontal polarization characteristics. However, there has been a problem of considerably degraded vertical-polarization-characteristics in the sleeve antenna.